Recently, as a next-generation NVRAM (Nonvolatile Random Access Memory) capable of operating at high speed to replace a flash memory, various kinds of device structures such as a FeRAM (Ferroelectric RAM), a MRAM (Magnetic RAM), an OUM (Ovonic Unified Memory) are proposed and a fierce development race is carried on in terms of implement high performance, high reliability, low cost, and process compatibility. However, each above memory device has good and bad points and it is far from an ideal “universal memory” having good points of a SRAM, a DRAM and a flash memory.
With respect to these existing techniques, a RRAM (Resistance Control Random Access Memory) comprising a variable resistor element in which an electric resistance is reversibly changed when a voltage pulse is applied has been proposed. The variable resistor element has an extremely simple laminated structure in which a variable resistor 2 is provided between two metal electrodes of an upper electrode 1 and a lower electrode 3 as shown in FIG. 1, and a resistance value between both electrodes can be reversibly changed by applying a voltage pulse between the upper electrode 1 and the lower electrode 3. A new nonvolatile memory device can be implemented by reading a resistance value in the reversible resistance changing operation (referred to as the “switching operation” occasionally hereinafter).
A method of changing an electric resistance reversibly by applying a voltage pulse to a perovskite material known for its colossal magnetoresistance (CMR) effect as a material of the variable resistor is disclosed in the following patent document 1 and non-patent document 1 by Shangquing Liu and Alex Ignatiev et al. in U.S. Houston University. This is an extremely epoch-making method in which while the perovskite material known for its colossal magnetoresistance effect is used, a resistance change over several digits can be provided at room temperature without applying a magnetic field. In addition, according to an element structure disclosed in the patent document 1, a crystalline praseodymium calcium manganese oxide Pr1−zCazMnO3 (referred to as the PCMO occasionally hereinafter) film that is a perovskite-type oxide is used as a material of the variable resistor. In addition, the fact that the switching operation can be provided in a binary transition metal oxide other than the perovskite material is disclosed in non-patent document 5.
As for now, although the switching operation principle of the variable resistor element is not clarified, the fact that a memory having a large resistance ratio can be implemented by forming a Schottky barrier between the electrode and the variable resistor is disclosed in the following non-patent document 2 by Sawa et al. in National Institute of Advanced Industrial Science and Technology.    Patent document 1: U.S. Pat. No. 6,204,139    Non-patent document 1: Zhuang, H. H. et al., “Novel Colossal Random Access Memory (RRAM)”, IEDM, Article No. 7.5, December 2002    Non-patent document 2: A. Sawa et al., “Hysteretic current-voltage characteristic and resistance switching at a rectifying n/Pr0.7Ca0.3MnO3 interface”, Applied Physics Letter, vol. 85 pp. 4073 to 4075, November 2004    Non-patent document 3: M. Imada et. al., “Metal-Insulator Transition”, Review of Moderan Physics 70, pp. 1039 to 1247, 1998, especially in Chapter 4 (pp. 1144 to 1245)    Non-patent document 4: G. Kotliar et al., “Compressibility Divergence and the Finite Temperature Mott Transition”, Physical Review Letters, Vol. 89, No. 4, pp. 046401-1 to 046401-4, 2002    Non-patent document 5: G. Dearnaley et al., “Electrical Phenomena in Amorphous Oxide Films”, Reports on Progress in Physics 33, pp. 1129 to 1191, September, 1970
However, in the resistance change random access memory proposed by the conventional invention and document, the principle of the switching operation has not yet clarified, and a control indicator of a process parameter to design the device has not clarified.
The present invention was made in view of the above problems and it is an object of the present invention to provide a variable resistor element provided with a variable resistor formed of a strongly-correlated material between two metal electrodes and changing an electric resistance between the metal electrodes when a voltage pulse is applied between the metal electrodes, in which a switching operation having a large resistance ratio between a low resistance state and a high resistance state of the electric resistance can be implemented by appropriately designing the metal electrode and the variable resistor based on a clear switching operation principle.